When an arbitrary controlled object is feedback-controlled, the general practice is to calculate deviation between a reference input value and a measured value acquired from the controlled object, compute feedback control from this deviation, and control the controlled object using an operation amount obtained by the computation, thereby performing the desired operation.
Feedback control aimed at digitation is performed, for example, by imputing a control target value generated by a signal generator or the like to a deviator. Further, load, displacement or the like sensed by a sensing amplifier is digitally converted and input to the deviator. The deviator computes the deviation between the control target value and the sensed value, a digital control unit performs digital computation based on the deviation concerned under a predetermined gain, and the operation of the controlled object is controlled using the computed control output (see Patent reference 1, for example).
The amplification factor (sensing range) of the aforesaid sensing amplifier is switched by a range switching control unit and the value sensed in the sensing range concerned is output to the deviator. In this apparatus, the switching of the sensing range stabilizes the feedback control and enables high-accuracy execution.
FIG. 1 is an explanatory diagram showing a structure example of an apparatus using conventional feedback control. This apparatus is configured to generate an output signal Y, which is an analog signal, based on a digitized reference input value X, for controlling a controlled object 107. This drawing shows only the portion that performs the feedback control.
The portion that performs the feedback control comprises a range switching unit 101, a deviation calculating unit 102, a control unit 103 that performs PI or PID control, a digital-analogue converter (hereinafter denoted DAC/analog output unit 104, a gain regulator 105, an analog-digital converter (hereinafter denoted ADC) 106, and the controlled object 107.
The range switching unit 101 is provided in advance with a number of input ranges, and upon receiving a reference input value X input from the outside, switches the magnitude of the reference input value X to an appropriate range in accordance with, as a specific example, the number of significant digits representing the value, thereby applying an appropriate gain for enlarging the number of significant digits of the reference input value X.
The reference input value X input to the range switching unit 101 becomes data having a number of significant digits within a prescribed range of the aforesaid input range. The number of significant digits of the data is the same as the number of significant digits of a measured value M output from the ADC 106. The range switching unit 101 outputs the data having the aforesaid number of significant digits as a setting value X′.
When the apparatus starts and firstly receives the setting value X′ as input, the deviation calculating unit 102 outputs to the control unit 103 a signal indicating to the effect that the setting value X′ does not have a deviation e, or a deviation e of “0” value, and the setting value X′.
The control unit 103 calculates an operation amount for the controlled object 107 corresponding to the setting value X′ including the deviation e; for example, an object generates a control signal for PI control or PID control. The input and output signals handled in the range switching unit 101, deviation calculating unit 102 and control unit 103 are digital signals.
Upon receiving the control signal representing the operation amount, the DAC/analog output unit 104 D-A converts the control signal to generate an analog signal that can control the operation of the controlled object 107, or a drive voltage for the controlled object 107, or similar, and outputs the result to the controlled object 107 as the output signal Y. Further, a controlled variable Z is input to the gain regulator 105 which constitutes a feedback path.
The gain regulator 105 converts the signal level of the controlled variable Z to generate an analog signal inputtable to the ADC 106. In other words, the signal level is converted to one suitable for the input dynamic range of the ADC 106.
More specifically, the gain regulator 105 selects from among its own predefined multiple measurement ranges a measurement range that enables input of the controlled variable Z output from the controlled object 107.
Next, the controlled variable Z input at this measurement range is subjected to the amplification (or attenuation) processing defined by that measurement range to convert the controlled variable Z to a signal level inputtable to the ADC 106.
The measurement range switching operation in the gain regulator 105 is performed synchronously with the input range switching at the time the reference input value X is input to the range switching unit 101. The predefined measurement ranges in the gain regulator 105 are, for example, defined to make the measurement range of each range the power of ten.
The ADC 106 A-D converts the analog signal input from the gain regulator 105 to generate the measured value M of prescribed bit length.
After the apparatus goes into operation, the measured value M is sequentially input to the deviation calculating unit 102 together with the setting value X′ output from the range switching unit 101, and the deviation e of the input setting value X′ at this time is calculated and output to control unit 103.
The control unit 103 uses the input deviation e to perform feedback control computation to generate a control signal indicating a operation amount taking the amount of feedback into account, which it outputs to the DAC/analog output unit 104.
The DAC/analog output unit 104 varies the value of the output signal Y in accordance with the control signal taking the feedback amount input from the control unit 103 into account and outputs this signal to the controlled object 107. The apparatus exemplified in FIG. 1 performs feedback control in this manner.
FIG. 2 is an explanatory diagram showing an operation example of the apparatus utilizing the conventional feedback control. This drawing shows the responsiveness of the ranges switched in the range switching unit 101, and here exemplifies the responsiveness in PI control. In the drawing, the horizontal axis represents elapsed control time and the vertical axis represents percentage of the number of significant digits representing the setting value X′ in the input range used in range switching unit 101, for example.
The depicted characteristic curves A(2), B(2) and C(2) represent responsiveness during operation under an input range used when the range switching unit 101 inputs a large reference input value X (input range of small gain), and the characteristic curve D(2) represents the responsiveness during operation under an input range used when a small reference input value X is input (input range of large gain).
Although PID control theoretically rests on the premise that the whole system including the controlled object is linear, an actual system generally includes some portion with nonlinear characteristics. For example, the controlled object itself, or when, as is often the case, semiconductor devices are used in the process of operation amount D-A conversion, the input-output characteristics (such as the base voltage-collector current characteristics) of the transistors or other semiconductor devices themselves are essentially non-linear, and since the characteristics during large input therefore differ greatly from the characteristics during small input, the response characteristics frequently vary considerably under the same PI control parameters.
In control processing using an input range corresponding to the aforesaid large value, when the level of the setting value X′ or the reference input value X is small in comparison with the characteristic curve A(2) or the characteristic curve B(2), as in the characteristic curve C(2), bit overflow occurs during the calculation of deviation e, the system gain of the apparatus operation including the feedback control diminishes, and the response of the apparatus with respect input slows down.
Thus, during operation under an input range using a large value, the rising characteristic of the apparatus deteriorates with the decrease of the setting value X′ or the reference input value X, as indicated by the rising characteristic curve E(2) indicated by a broken line in FIG. 2.
In contrast, in operation under an input range corresponding to a small value, in the case of inputting a reference input value X of a small value similar to the characteristic curve C(s), a large gain is applied that increases the number of significant digits of the setting value X′. As a result, the number of significant digits of the deviation e obtained with respect to the setting value X′ also increases.
When an input range corresponding to a small value is used as in the foregoing, the characteristic curve D(2) shown in FIG. 5 is obtained, so that apparatus responsiveness (rising characteristic) becomes better and feedback control convergence becomes faster than in the characteristic curve C(2).
Thus for improving the rising characteristic (control accuracy) and the like in conventional digitized feedback control, it is important to switch to an appropriate input range when inputting the reference input value X.